The Phoenix Collegiate sent a letter to parents on Thursday which says the pupil has not been in school since they developed symptoms.
It also says 20 students and one staff member have been sent home to self-isolate.
It comes after a survey showed more than four in five schools in England currently have children away from class because they cannot access a coronavirus test.
Mike Smith, headteacher, says in the letter: "I am writing to inform you that one of our Year 11 students has tested positive for coronavirus (COVID-19).
"The individual has not been in school since they developed symptoms and has been self-isolating at home, as per government advice. They have since accessed a test and have tested positive.
"The individual’s close contacts have been identified. Close contacts are: those who have been in face-to-face contact with the individual within one metre, including being coughed or sneezed on and skin-to-skin contact, those who have been in extended close contact (i.e. more than 15 minutes) with the individual within two metres and those who have travelled in a small vehicle (e.g. a car) with the individual.
"Following advice from the DfE’s dedicated NHS advice team, 20 students and one staff member identified as close contacts of the individual have been sent home and advised to self-isolate for 14 days.
"You will have been contacted directly if your child needs to self-isolate – if you have not been contacted, you do not need to take any action at this point. If your child is in the affected group, they will not be penalised for non-attendance and remote education will be provided throughout the period they are at home.
"The other members of your household are not required to self-isolate unless your child subsequently develops symptoms.
"If your child develops symptoms, they will need to self-isolate for 10 days from the day their symptoms began, and the other members of your household for 14 days from this day."
The letter adds: "Finally I would like to reassure you that we take any suspected and confirmed cases of coronavirus very seriously.
"As soon as the individual developed symptoms, they were immediately sent home and a deep clean of any areas they were in was carried out.
"The whole school has since been deep cleaned.
"I understand that you may find this news worrying, so please get in touch with your child’s Head of House should you wish to discuss any concerns.
"I am sure you will all join me in wishing the individual a safe recovery."